Kurma avatar in Shatapatha brahmana and Taittariya aranyaka is referred to as Sacrifice and yonder Aditya which are both ephithet for Vishnu.
Kurma avatar and Vishnu are both mentioned as the Yonder Aditya :
1:
स यः कूर्मोऽसौ स आदित्यः । अमुमेवैतदादित्यमुपदधाति ।
“That which is Kūrma—that is yonder Āditya [the Sun]. It is yonder Āditya that he thereby places down.”
7.5.1.6
स यः स विष्णुः स यः स यज्ञः ।
असौ स आदित्यः ॥
2:
“That which is Viṣṇu is that which is Yajña; that [yonder] is Āditya.”
14.1.1.6
As well as sacrifice :
1:SB 1.6.2.3
ते पुरोडाशं कूर्मं भूत्वा चरन्तमपश्यन् ।
ते ह ब्रुवन् — एष वै यज्ञ इति ॥
“They saw the puroḍāśa having become a tortoise and moving about.
They said: ‘This indeed is the sacrifice.’”
2:
यज्ञो वै विष्णुः ।
yajño vai viṣṇuḥ
“Yajña indeed is Viṣṇu.”
In Taittariya aranyaka it is directly mentioned that Kurma which is the embodiment of sacrifice is the purusha who created Prajapati
- There after from the waters , kurma or tortoise crawled therein. Prajapati addressed the kurma: have you emerged from my skin and flesh. Kurma replied in the negative and corrrected Brahma by saying that he existed earlier as he was the that Purusa with the Purusatvam; the tortoise then assumed form of Purusa with thousand heads and thousand feet and the thousand eyes flashing from the waters. Then the Creator Prajapati exclaimed indeed, you existed well before I was born; since you were the first , you created the universe well before me!
And also Kurma also shares similar characteristics of Vishnu who prevaded the three worlds with three steps
तस्य यदधरं कपालम् । अयं स लोकः । तत्प्रतिष्ठितमिव भवति प्रतिष्ठित इव ह्ययं लोकः । अथ यदुत्तरं सा द्यौः । तद्व्यवगृहीतान्तमिव भवति व्यावगृहीतान्तेव हि द्यौः । अथ यदन्तरा तदन्तरिक्षम् । स एष इमे एव लोकाः । इमान् एवैतल्लोकानुपदधाति ॥
7.5.1.2
pra tad viṣṇuḥ stavate vīryeṇa
mṛgo na bhīmaḥ kucaro giriṣṭhāḥ |
yasyoruṣu triṣu vikramaṇeṣv
adhikṣiyanti bhuvanāni viśvā ||
“That Viṣṇu is praised for his might,
like a fearsome roaming beast dwelling upon the mountains,
in whose three wide strides all the worlds dwell
In Mahabharata and Ramayana. He appeared to support the gods during samudra manthan